The invention relates to the removal of copy protection signals from a video signal and, in particular, to a method and apparatus for disabling such copy protection signals by selectively modifying the envelope of an RF carrier signal which has been modulated by the copy protection signals. The invention is applicable in various embodiments for restoring the copiability of video signals which have been prerecorded in a recording medium, have been transmitted via satellite, are generated by a television camera, and/or are recovered via transmission over a television channel or cable system. The invention further relates to the modification of a RF carrier signal, which is modulated by a standard video signal which does not contain copy protection signals, to add copy protection signals to the RF carrier signal in the RF domain.
As evidenced by the above cross reference to related applications and patents, there are various techniques known in the art for not only inhibiting the making of acceptable video recordings, but also for defeating such copy protection techniques to allow the video signal to be recorded, that is, copied in some manner. That is, under certain circumstances, there is a need to disable the copy protection system to permit recording of copy-protected recordings, such as, for example, during authorized and permitted copying or for studio editing purposes. As may be derived from the above-mentioned references, there are several basic techniques typically applied to provide copy protection of recorded video signals as well as off-air or transmitted video signals. Likewise, there are several basic techniques typically applied to defeat or otherwise remove the various copy protection techniques used with any of the various sources of video signals.
Exemplary of a copy protection technique is the subject matter of the U.S. Pat. Nos. 4,631,603, 4,819,098 and 4,907,093 of previous mention in the Cross Reference, the disclosures thereof being incorporated herein by reference, which concern modifying a video signal such that a television set produces normal video pictures from the modified signal, but a videotape recording made from the modified signal produces unacceptable pictures. These copy protection techniques rely on the addition of pseudo-sync signals, similar to the normal sync pulses, and/or the addition of high level positive pulses (hereinafter referred to as automatic gain control or AGC pulses) following the trailing edges of a percentage of the normal and pseudo-sync pulses. Such copy protection signals are hereinafter also referred to as "anti-copy pulses and/or signals." The pseudo-sync pulses, which extend from the blanking signal level to the normal sync tip level of the video signal, generally are added to the video signal during selected lines of the vertical blanking interval. Each of the pseudo-sync pulses may be followed by a respective high level AGC pulse of previous mention to provide pairs of pseudo-sync and AGC pulses which then are inserted at certain locations in the video signal, for example, in the vertical blanking interval or in the video lines immediately adjacent the vertical blanking interval. Copy protection is provided by the exaggerated voltage difference between the sync and/or pseudo-sync tip and the respective high AGC pulse level which, in turn, causes the automatic gain control (AGC) system in, for example, a videotape recorder or record VCR, to make an erroneous assessment of the video signal level. It follows that the AGC system produces a gain correction which results in reducing the signal recorded on videotape to a prohibitively low level. Thus, during playback of such a recording, the resulting recovered video signal will generate unstable pictures of unacceptably poor quality on a normal television set.
A further example of a copy protection technique is disclosed in the U.S. Pat. No. 4,577,216 of previous mention in the Cross Reference, which disclosure is incorporated herein by reference, and which concerns modifying a color video signal such that a television set produces a normal color picture while a videotape recording of the modified color video signal exhibits annoying color interference in the form of horizontally arranged color bands, hereinafter referred to as a "color stripe" effect. Such a color stripe effect also is considered to be, and is referred to herein as "anti-copy pulses or signals" or copy protection signals. To this end, the color burst signal integral with the video signal is phase modulated in any of various manners such as, for example, by a random, or pseudorandom noise signal, to produce a modified color burst signal having deliberate phase errors. It follows that the circuitry of a television set inherently ignores such phase modulation to produce an acceptable color picture, but the velocity error, or color stabilizer, correction circuit in a VCR will attempt to correct the introduced phase errors, as is disclosed in the above patent U.S. Pat. No. 4,577,216. As discussed therein, the velocity error correction circuit will cause the error to be transferred from the color burst signal to the chrominance signal of the video signal, thereby causing severe and objectionable color noise, that is, the color stripe effect, in the video color picture.
Exemplary of techniques for defeating or removing the effects of the copy protection systems of previous discussion above, are those disclosed in the U.S. Pat. Nos. 4,695,901, 5,157,510 and 5,194,965 of previous mention in the Cross Reference, and whose disclosures are incorporated herein by reference. The subject matters of these references are concerned with defeating or disabling the copy protection techniques of previous discussion which employ pseudo-sync and/or AGC pulses as anti-copy signals to inhibit copying of the video signal. To this end, the pseudo-sync and/or AGC pulses effectively may be removed or disabled by first determining the timing locations of the anti-copy pulses and then by level shifting, narrowing the width or otherwise increasing the frequency content, and/or by attenuating, replacing or otherwise modifying, at least some of the pulses by an amount that causes the VCR circuit to fail to respond to the anti-copy pulses which provide the copy protection. The AGC pulses may be removed by replacing them with another selected signal or by selectively blanking the video signal during predetermined time periods following the occurrence of sync pulses. Further, removal of pseudo-sync pulses may be accomplished by selectively clipping the negative-going pseudo-sync pulses in response to the detection of the pulses. Accordingly, such copy protection removal techniques contemplate various means which replace, suppress or otherwise modify the video signal such that, because of characteristics of a VCR, the anti-copy signals become less effective or are nullified to no longer prevent unauthorized copying.
Typical of techniques for defeating or removing the copy protection systems which generate the color stripe effect, are those disclosed in the U.S. Pat. No. 4,626,890 and copending U.S. application Ser. No. 08/438,155, filed May 9, 1995 of previous mention in the Cross Reference, and whose disclosures are incorporated herein by reference. The techniques disclosed in the latter patent and application rely on first determining the video line locations containing the color stripe bursts, either from prior analysis and programming in location to, for example, a programmable memory, or by sensing each color burst which contains a color stripe on a line-by-line basis using, for example, a phase detector circuit. Secondly, the references are concerned with defeating the color stripe effect by replacing the modulated color burst signal with a regenerated color burst subcarrier component, modifying the color stripe burst or otherwise modifying the video signal (for example, modifying the horizontal sync pulse preceding the color stripe burst), eliminating some of the color stripe bursts, attenuating the color stripe bursts in amplitude or duration, and/or removing, phase shifting or attenuating a portion of each color stripe burst or most of the color stripe bursts. Alternatively, the color burst signal, and thus any color stripe burst, can be replaced throughout the active television field with respective correct color burst signals, which thus permits subsequently recording an acceptable copy.
As may be seen by review of the above related references, the techniques and apparatus for providing copy protection as well as the techniques and apparatus for defeating the anti-copy signals of the copy protection systems, all operate in the baseband video domain using baseband video signals to derive the anti-copy pulses. Furthermore, the selected modifications of the anti-copy pulses, including the color stripe effect, to disable the copy protection systems to thereby allow the making of acceptable video signal recordings, also are performed in the baseband video domain.
The techniques for defeating copy protection in the baseband domain have the disadvantage of requiring the use of two separate leads containing an audio and a video signal, respectively, whereas the RF carrier based system of the present invention for defeating copy protection techniques requires a single lead carrying the RF carrier signal corresponding, for example, to a luminance signal. Further, when defeating copy protection signals in an off-air, cable, etc., environment, a copy protection defeating system in the baseband domain requires the added complexity of a tuner demodulator circuit to permit a recordable copy, whereas the RF carrier domain system does not.